I'm no expert, but I have a brand new Pavoni and the boiler looks a bit like that. A non uniform brownish, with varying degree of darkness.
From what I've read this should be normal, but it is also said that descaling with citric might remove a natural protection the copper creates for itself (when oxidizing, I guess), and possible lead to further corrosion. Just google up copper corrosion and oxidation and you will find a lot of information.
All-in-all, yours to my eye looks good. But, let's see if someone more expert will chime in
P.S. I guess one could polish the boiler from the inside
Not sure how useful that would be though.